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Ethan Buhr, Russell Van Gelder; The photoentrainment of circadian clocks in the mammalian retina. Invest. Ophthalmol. Vis. Sci. 2013;54(15):403.
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© ARVO (1962-2015); The Authors (2016-present)
The mammalian retina has the ability to synchronize the local circadian rhythms of gene expression within retinal cells to light:dark cycles. Remarkably, this photic entrainment is maintained ex vivo. Rod opsin, cone opsin, and melanopsin are required, alone or in combination, for the synchronization of the behavioral rhythms of the whole organism. We sought to determine which if any of these photopigments are required for entrainment of local circadian rhythms.
The PER2:LUCIFERASE knock-in mice contain a luminescent reporter of a circadianly regulated gene which allows for the measurement of rhythms from cultured tissue. Retinas from these mice were cultured for weeks at a time and displayed robust luciferase rhythmicity without retina pigment epithelium or an exogenous supply of retinoid. This was also true of mice carrying null alleles of Opn4 (melanopsin), Pde6b (rd1), or a combination of the two mutant alleles. LED lights with peak emissions at 375nm, 417nm, 475nm, and 530nm were used to test the photic sensitivity of these retinas. Wheel-running activity of Opn4-/-;rd1/rd1 mice was measured in the presence of a light:dark cycle to confirm the lack of photic entrainment of behavior in these animals.
Retinas of mice from all four genotypes (wild-type, Opn4-/-, rd1/rd1, and Opn4-/-;rd1rd1) were synchronized to light:dark cycles ex vivo. Wild-type pituitary and ear skin cultures were not synchronized by these same light:dark cycles. Wild-type retinas did not entrain when covered by foil or red acetate. The phase of circadian rhythms of retinas was sensitive to short wavelength light:dark cycles (417nm), but not to longer wavelengths of equal photon flux (530nm).
These results demonstrate that neither rods, cones, nor melanopsin are required for the synchronization of retinal circadian rhythms to light:dark cycles. This suggests the existence of a separate, short-wavelength photoreceptor or set of photoreceptors that mediate local entrainment of the retinal clocks to light-dark cycles.
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